In the manufacture of a silicon single crystal by the Czochralski method (CZ method), a vitreous silica crucible is used. In the CZ method, silicon raw material is charged into the vitreous silica crucible and heated to melt. Then, a seed crystal is dipped into this silicon melt, and the seed crystal is gradually pulled up while rotating the crucible to grow a single crystal. In order to manufacture a silicon single crystal at low cost, it is necessary to improve the single crystal yield at one time of pulling process. For this purpose, it is necessary to use a large capacity crucible capable of holding a large amount of raw material.
In the preparatory stage of pulling the silicon single crystal by the CZ method, raw material blocks of polycrystalline silicon are filled in the vitreous silica crucible in advance. This filling operation is performed manually, and the raw material blocks are packed carefully one by one so that no impact will be applied to the crucible. At this time, the operation takes about 1 hour in the case of CZ pulling of a silicon single crystal having a diameter of 300 mm, for example. The vitreous silica crucible is very delicate and a breakage or a defect is easily generated. This is because the vitreous silica crucible is easily broken when a large amount of raw material is introduced vigorously. Further, in order to obtain as large a silicon single crystal ingot as possible at one time of pulling process, it is necessary to first fill as much raw material as possible in the crucible without gaps. For that purpose, a careful filling operation is required in consideration of the size, the shape and the like of the raw material blocks. Patent Document 1 describes a method of filling raw material blocks of polycrystalline silicon. In this filling method, the first layer of polycrystalline silicon blocks is formed in the crucible, and the second layer of polycrystalline silicon blocks is formed on the first layer. The height of the first layer is made lower than the height of the melt surface after all the polycrystalline silicon raw material has been melted, and the outer periphery of the second layer is separated from the inner peripheral surface of the crucible.
However, even if a careful filling operation is performed manually, a phenomenon where the vitreous silica crucible is broken suddenly during filling the small pieces of polycrystalline silicon is seen. When the vitreous silica crucible is broken, not only the crucible itself cannot be used, but also the filling operation of the raw material that has been done so far is wasted, resulting in a very large loss in terms of cost and man-hour. Such breakage is hard to be prevented completely, but it is possible to know the type and condition of an easily broken crucible, and the problem can be avoided by treating the easily broken crucible as a defective product. For that purpose, it is necessary to inspect the vitreous silica crucible in a condition as close as possible to the actual status of use.
As an inspection method of a vitreous silica crucible, for example, Patent Document 2 describes an optical non-destructive inspection method of inspecting bubbles existing in the inner surface of the vitreous silica crucible by using optical detecting means and image processing means. Further, Patent Document 3 describes a crucible inspection method of inspecting only the impurities contained in the outermost surface layer of the crucible inner surface, by specifying the impurity component from the wavelength and intensity of the fluorescence generated by laser light irradiation and calculating the content of the impurities. In addition, Patent Document 4 describes a method of inspecting an abnormal site in a vitreous silica crucible. In the method, at least one of an infrared absorption spectrum and a Raman spectrum is measured at a measurement point on the crucible inner surface, and whether or not an abnormal site such as brown ring or the like is generated at the measurement site is judged based on the obtained spectrum. Further, Patent Document 5 describes a method of obtaining the hardness of a silica sintered body crucible from the force of a release point after a load was continuously applied. In Patent Document 5, the hardness of a crucible is defined by pressing force.